Early 60's DR Reverb Problem

[fusionbear]

A customer brought me a DR that he bought new in the early 60's that had a bad reverb tank. The input was open (in the inductor). So I replaced it with the same type (Low Zin, HighZout) but now it begins to squeal if I turn it up past 3-4 and there is excessive hard break up in the vibrato channel even with the rev at 0. As soon as I unplug the tank, problem goes away. Have already tried to replace the 12AT7 and 12AX7 tubes, but to no avail. Any Ideas?

Thanks....

[Andy Le Blanc]

hopefully youve kept the old tank..... and hopefully you can track
down a shematic..... heres why...
the info on a reverb tank includes :

input impeadance
output impeadance
decay time
connection type
lock and mounting

you need to be aware of the connectors and mount style

but its the connector type the can cause your symptoms
especialy if the amp is fine other wise.... that means how the unit is
connected to ground... you can easily create a loop and make your amp
squeal...

there is good info at aiken amps in the tech info under data sheets
for accutronics verb tanks..... you can also check the old tank and
a schematic if you can get one to double check how the in ands out of the unit are grounded

[Phil_S]

Did you try reversing the cable?

[Firestorm]

Make sure you have a type 4AB3C1B tank. That will insure that the impedances and grounding is correct for a Fender application. The cables to the tank itself must be shielded, of course, but if you also have the two-button footswitch, you must make sure the reverb lead is the shielded one. The "Vibrato" lead is unshielded. The cable out to the footswitch is directly connected to the grid of the reverb recovery tube so there's plenty of opportunity for noise it it isn't shielded.

But none of this should contribute to a bad breakup problem. The Reverb control is a "mix" control, so when it's set on "0," the wiper of the pot is at ground. Take a look at V4: the reverb recovery amp and the reverb mix amp share a cathode resistor and a bypass cap. Make sure the cap is good and the resistor within specs. Also check the 220K resistor that goes from the Reverb pedal jack to ground. This is the grid load resistor for the reverb recovery amp and might throw the tube off if something's screwy.

[fusionbear]

Make sure you have a type 4AB3C1B tank. That will insure that the impedances and grounding is correct for a Fender application. The cables to the tank itself must be shielded, of course, but if you also have the two-button footswitch, you must make sure the reverb lead is the shielded one. The "Vibrato" lead is unshielded. The cable out to the footswitch is directly connected to the grid of the reverb recovery tube so there's plenty of opportunity for noise it it isn't shielded.

But none of this should contribute to a bad breakup problem. The Reverb control is a "mix" control, so when it's set on "0," the wiper of the pot is at ground. Take a look at V4: the reverb recovery amp and the reverb mix amp share a cathode resistor and a bypass cap. Make sure the cap is good and the resistor within specs. Also check the 220K resistor that goes from the Reverb pedal jack to ground. This is the grid load resistor for the reverb recovery amp and might throw the tube off if something's screwy.

Thanks for the tip. The 220k resistor went open.